The present invention generally relates to the testing of a capacitor without disconnecting the capacitor from the circuit in which it is connected.
Capacitors have always had a relatively high failure rate when compared to other electronic components and thus require frequent testing when a problem arises in the operation of an electronic circuit. While recent improvements in the processes used to manufacture capacitors have improved the reliability of present day capacitors, they still have a relatively high failure rate. Moreover, the use of capacitors has been increasing, wherefore the need to accurately and quickly determine if a capacitor has failed is of great importance.
It has long been believed that capacitors could not be reliably tested without first removing the capacitors from the associated circuitry because of the effects that other circuit components could have on the test measurement. Therefore, in order to accurately test capacitors, technicians have had to remove the capacitors from the circuits in which they were connected, then make the necessary test measurements, and if the capacitors were determined to be good, to reconnect them into the circuit. Of course, if they were determined to be bad, they were replaced by good capacitors, Especially if the capacitor is connected to a printed circuit, this is a very time consuming and delicate procedure if damage to the associated printed circuit is to be avoided. Indeed, the increasing use of miniature surface mounted capacitors has exacerbated the problem since these capacitors are extremely small and fragile. Accordingly, special tools are required to handle these miniature capacitors, and many technicians do not have such tools or the expertise to use them.
Various methods and instruments have been used in the past to measure the capacitance of capacitors. For example, impedance bridges have been used to apply a signal of known amplitude and frequency to a capacitor under test, but such methods and instruments cannot be used to measure the capacitance of an in-circuit capacitor because of the effects of the associated circuits on the measurements. Moreover, other common failure modes of a capacitor are not detected by the use of these bridges. Other methods and instruments have been used to measure the ESR, equivalent series resistance, of an in-circuit capacitor at low voltage levels so that the effects on the measurements are minimized. However, such instruments do not show if the capacitance value is within tolerance limits or if there is any leakage internally of the capacitor or in the associated circuit.
Other capacitor testing systems have also been proposed, but they too have been ineffective to make accurate measurements of the important characteristics of a capacitor under test. Therefore, it would be desirable to provide a method and apparatus for analyzing the characteristics of a capacitor while that capacitor is connected in circuit with other electric components.
Briefly, in accordance with the teachings of the present invention, a capacitor can be tested while connected to other electronic components in an associated circuit and such test will determine whether the capacitor is good or bad or in some cases should be removed from the associated circuit for further testing. This test method and instrument makes a sequence of tests on a capacitor while the capacitor is connected to other electronic components or even disconnected therefrom, which tests include a DC capacitance value measurement, a measurement of the DC current paths in parallel with the capacitor, a measurement of the equivalent series resistance of the capacitor, and an AC impedance measurement of the capacitor and its associated circuitry. It is the sequencing of these four tests and the analysis of the individual test results that enable the technician to determine if a capacitor is either good or bad. In some rare instances, the test results may be questionable and in those cases the technician is advised to disconnect the capacitor under test from the associated circuitry and to repeat the test.
The instrument described in the detailed description to follow herein automatically makes the above-described tests one after another and analyzes the resulting measurements to provide a visual indication to the technician as to whether the capacitor under test is good or bad.